The internal leakage of gear pumps directly determines the volumetric efficiency of gear pumps. Through the analysis of the structure of the gear pump, it is found that there are three main ways of internal leakage: (1) through the clearance between the end face of the gear and the side plate; (2) through the radial clearance between the top circle of the gear and the inner cavity of the pump; (3) contact clearance between the gear mesh.

The axial clearance between the end face and the side plate of the gear is determined by the structure of the gear pump, and the contact clearance between the gears is determined by the precision of the gear of the pump. The leakage between the top circle of the pump gear and the radial clearance in the pump body is closely related to the selection of the gear parameters.

The end clearance has the greatest influence on flow leakage. This is mainly due to the short and wide leakage path through the end clearance. The data show that the leakage through the end clearance can account for 75% - 80% of the total leakage. When the end clearance increases by 0.1 mm, the volumetric efficiency decreases by 20%, and when the radial clearance increases by 0.1 mm, the volumetric efficiency decreases by 0.25%. The end clearance between the axial end face of the rotor and the pump cover is about 0.02-0.12 mm. The leakage is directly proportional to the three power of the pressure. The larger the gap, the higher the pressure, the more leakage, and the lower the volumetric efficiency. This is especially true for low viscosity diesel pumps. Small clearance can improve volumetric efficiency, but the clearance is too small, processing accuracy requirements are high, production costs increase, but also easy to appear stuck phenomenon. For high pressure pumps, the design method of end face clearance compensation structure must be adopted. On the premise of guaranteeing the rotor flexible operation, machining accuracy, assembly and flexible operation, the end clearance should be reduced as far as possible.